1. Field of the Invention
The present invention generally relates to an image forming apparatus and, more particularly, to an image forming apparatus including a recording head that ejects droplets.
2. Description of the Related Art
Inkjet recording apparatuses are known as liquid-ejection-recording type image forming apparatuses, such as printers, facsimiles, copiers, plotters, and multifunction peripherals, that employ a liquid ejection head that ejects, for example, droplets as a recording head.
It is known that nozzle maintenance of a liquid ejection head of such an image forming apparatus can be performed by applying a slight-vibration generating waveform to a pressure generator of a nozzle (hereinafter, “non-ejection nozzle”) from which a droplet is not to be ejected, thereby vibrating a meniscus of liquid in the nozzle in a manner that does not eject a droplet.
Known methods for applying such a slight-vibration generating waveform include a method of generating a common driving waveform containing an ejection waveform and the slight-vibration generating waveform and selecting one of the waveforms based on image data (ejection data). However, this method is disadvantageous in that because the driving waveform contains both the ejection waveform and the slight-vibration generating waveform, the driving waveform has a large waveform length and therefore is less preferable for speed-up.
A technique for solving this problem is disclosed in Japanese Patent Application Laid-open No. 2007-276287. In this technique, three voltage sources (a high voltage source, a medium voltage source, and a low voltage source) are connected to a piezoelectric element in a manner that allows applying a selected one of voltage outputs to the piezoelectric element or, in other words, applying a ternary digital driving waveform to the piezoelectric element. The three voltages are set such that a difference between the high voltage and the medium voltage differs from a difference between the medium voltage and the low voltage level. Slight vibrations, by which no droplet is ejected from a recording device, are generated by switching the connection so as to apply either a waveform ranging between the medium voltage and the high voltage or a waveform ranging between the medium voltage and the low voltage depending on an environmental temperature.
Japanese Patent No. 4259741 discloses an apparatus that includes a circuit for generating a first driving waveform that causes an ink droplet to be ejected in one drive period and a second driving waveform that vibrates an ink meniscus without causing an ink droplet to be ejected in time series, and a unit that selects the first driving waveform according to a print signal and selects, independently of the print signal, the second driving waveform according to a meniscus-vibration selecting signal generated every n (n is an integer greater than one) drive periods and applies the waveforms to a plurality of electrodes simultaneously.
However, the configuration disclosed in Japanese Patent Application Laid-open No. 2007-276287 that selectively applies one of the voltages of the plurality of voltage sources to the recording device has a problem that when adopted in an apparatus including a plurality of recording heads, if optimum slight-vibration generating waveforms vary due to variation among the recording heads, as many voltage sources as the recording heads become necessary. This leads to an increase in apparatus size.
The configuration disclosed in Japanese Patent No. 4259741 that allows selecting either the ejection waveform or the slight-vibration generating waveform separated from the common driving waveform has a problem that the apparatus needs to include two driving source systems specially for this driving scheme, which increases cost of the apparatus. Furthermore, the apparatus needs to include additional wiring for this driving scheme, which makes it difficult to miniaturize the apparatus.
In view of the problems mentioned above, there is needed to solve at least part of the problems and to configure an apparatus to be capable of generating slight vibrations without an increase in size of the apparatus.